The magnetic disk memory has become the dominant means of auxiliary storage of data in data processing systems. Although access times are still on the order of several milliseconds, once access is completed, data rates are quite high. Increased bit densities permit the storage of an astonishing amount of data in a relatively small volume.
In such devices a plurality of disks are usually mounted for rotation on a motor driven spindle. A carriage carries a plurality of arms, the free end of each carrying a transducing head placed in reading and writing proximity to a particular disk recording surface. The carriage is usually driven back and forth in a linear fashion by a linear motor or voice coil allowing the heads to scan radially across the disk surfaces to access the individual circular, concentric, sequentially numbered tracks. Recent important improvements in bit density and access time as a consequence of increased radial track density result from designating a surface of one disk, containing prewritten head positioning data, as a servo surface. A servo system controls the position of a servo head with respect to servo tracks on the servo surface thereby correctly positioning the carriage so as to allow the remaining heads to accurately address desired tracks on their respective recording surfaces. The servo tracks are also circular and concentric on the servo surface and are sequentially numbered. Each servo track along with its related data tracks form a group of tracks called a cylinder, each cylinder in essence defining a single carriage position. When it is desired to shift the heads from one cylinder to another in performing a so-called "seek" operation, the servo head is simply shifted to the corresponding destination track, allowing the selected data heads to reference the tracks in the desired cylinder. While the servo head is moving it generates a servo pulse as it crosses each servo track.
It is well-known that it is necessary to accurately control the velocity of the carriage during a move between track cylinders to assure rapid and accurate shifting between cylinders. This requires measuring instantaneous, or nearly instantaneous, velocity of the carriage. One common way to measure carriage velocity employs a magnetic speed transducer which is mounted within the linear motor and whose output signal is functionally related to the armature velocity. U.S. Pat. No. 4,031,443 (Droux, et al) discloses this type of mechanism. U.S. Pat. No. 4,068,267 (Inouye) employs the less conventional approach of differentiating the signals from the servo, in effect relying on the slope of the pulses which comprise the servo signal, to determine velocity. U.S. Pat. No. 4,068,269 (Commander, et al) describes apparatus which uses a similar approach to measure velocity.